The present invention relates to electrostatic plotters and, more particularly, to a humidifying system for maintaining the moisture content of the paper at a preferred level in an electrostatic plotter having a paper supply compartment rotatably holding a horizontal roll of plotting paper where the paper passes from the roll and through the supply compartment to a separate printing compartment comprising, powered humidifier means including a tank for holding a supply of water to be humidified for supplying cool water vapor at an outlet opening thereof; an adjustable humidistat disposed in the paper supply compartment and operatively connected to turn the humidifier means on and off as a function of the humidity in the paper supply compartment; and, a manifold horizontally disposed across the inside of the paper supply compartment, the manifold having a closed end and an opposite end connected to the outlet opening of the humidifier means, the manifold additionally having a plurality of spaced outlet holes through sidewalls thereof along the length thereof for water vapor introduced into the manifold by the humidifier means to emerge therefrom.
The need to control moisture and humidity in electrostatic printing processes and apparatus is well known and documented. For example, the 1967 patent of Schulze, et al. (U.S. Pat. No. 3,349,221) and the 1972 patent of MacDonald, Jr. (U.S. Pat. No. 3,677,632) are both directed to heating such apparatus for the purpose of eliminating excess moisture in overly moist environments. The 1972 patent of Verderber (U.S. Pat. No. 3,634,077) is directed to a system as shown in simplified form in FIG. 1 hereof where cut sheets of paper 10 employed in an electrostatic copying process are stored and fed from an environmentally controlled compartment 12. The compartment 12 contains a thermostatically or humidistat controlled heater 14 for eliminating moisture above an upper limit in essentially the same manner as the Schulze, et al. and MacDonald, Jr. apparatus. Verderber, however, additionally makes provision for adding moisture to the compartment 12 when the humidity therein falls below a lower limit. In the Verderber apparatus, there is a humidifier 14 connected to a blower 16 controlled by a humidistat 18. The air content of the compartment 12 is circulated by the blower 16 through the humidifier 14 when additional humidity is needed between an inlet opening 20 and an outlet opening 22.
The Verderber apparatus apparently works for its intended environment because of certain limitations on that environment and the paper employed in the associated process. As the Verderber patent states, "It is important to note that the size of the chamber 12 must be kept to a minimum, preferably not in excess of 3 cubic feet, inasmuch as an air volume in the chamber in excess of this amount would make maintenance of humidity conditions much more difficult, and hence could possibly involve the use of equipment whose size and cost would be out of scale with the size and cost of the usual copying machine, so as to render the combination of the present invention impractical." It should be noted in this regard that one of the important aspects of the Verderber patent is the fact that the power is always on to the environmental control equipment and the paper for future use is stored in the controlled compartment 12 as well. Verderber is also concerned with standard copy paper which comes in reams of cut sheets typically 81/2 by 11 inches. The moisture absorbing qualities of such paper are entirely different from other forms of copy paper.
In the field of X-Y plotters driven by computer, roll paper electrostatic plotters are gaining in popularity because of their ability to produce large, detailed drawings without the necessity of employing a liquid ink pen to draw all the lines as in pen plotters. Such an electrostatic plotter as known in the art is shown in simplified form in FIG. 2 where it is generally indicated as 24. There is a separate paper compartment at the rear of the plotter which is accessed through a hinged door 28. A roll 30 of paper 32 is rotatably supported on its ends by support bars 34. The paper 32 is fed from the roll 30 over guide rollers 36 and through a slot 38 into the printing compartment 40 where electrostatic printing apparatus 42 controlled by the control circuits 44. After the plot is printed onto the paper 32, it is fed over additional guide roller 36' out of the printing compartment 40 where each plot is cut off for use.
The paper 32 has certain qualities which make it unsuitable for use of a humidity control system such as that of Verderber--in part for the very reasons quoted above; that is, the roll 30 is typically three feet in length and tightly wound to a diameter of approximately six to eight inches. Accordingly, its ability to absorb moisture on the roll is quite limited. Any attempt to employ the Verderber approach would, in Verderber's words, "involve the use of equipment whose size and cost would be out of scale with the size and cost of the usual copying machine, so as to render the combination . . . impractical".
Because of the characteristics of the process and the equipment, what is required is a humidity source which can contact the paper 32 in a homogeneous manner principally in the portion between the roll 30 and the slot 38. This portion remains exposed for the duration of the plotting process--which, for a detailed plot, can take several minutes. The portion is then fed into the printing compartment 40 to be replaced by a next exposed portion. The typical plotting environment for such equipment is a high quality computer room having an air conditioning system. Excess moisture is not the problem, only an insufficiency of moisture in the paper. The paper employed is characterized by it ability to rapidly absorb moisture to which it is exposed. It is also characterized by the need to maintain the humidity at about the 50% to obtain good plots where there are areas of high line density. When the humidity falls below this level, the paper is incapable of maintaining a uniform electrostatic charge. As a result, areas of black become gray and lines lose continuity and contain gaps.
Initial attempts by the inventor herein to create a corrective environment took the form depicted in FIG. 2. An enclosure 46 of plastic similar in design and construction to those employed to abate noise in computer printers, and the like, was placed over the entire plotter 24. A piezoelectric humidifier 48 was connected by a duct 50 to conduct cool water vapor (as produced by an ultrasonic device within the humidifier 48) into the enclosure 46. A humidistat 18 was positioned in the enclosure 46 and connected to control the humidifier 48. This arrangement worked with respect to solving the moisture/humidity problem; but, the enclosure 46 was large, heavy, and cumbersome to work with. Aesthetically and operationally, it was undesirable for a commercial environment.
Wherefore, it became the object of the present invention to provide a system for providing a source of moisturizing humidity to the paper of a roll paper electrostatic plotter which is simple, inexpensive, effective, and neither aesthetically nor operationally undesirable for incorporation into such plotters as produced and sold by the assignee hereof.
Other objects and benefits of the present invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.